The studies outlined in this grant are designed to gain further knowledge of the biochemistry, physiology, and clinical importance of the platelet Alpha-granule protein Platelet Factor Four (PF-4). The specific aims are to: 1) define the molecular mechanism of PF-4 binding to heparin and other sulfated glycosaminoglycans (GAG); 2) study the binding, endocytosis, and intracellular degradation of PF-4 by vascular endothelial cells; 3) define the domains of the PF-4 molecule that are involved in antibody recognition and GAG binding; 4) evaluate the possibility that PF-4 can be used to localize GAG in tissues; and 5) study the effect of PF-4 on heparin-catalyzed and heparin-independent coagulation reactions. The studies will exploit some exciting new information that localizes both the GAG binding site and the major antigenic domain of the molecule to the carboxy terminus and demonstrates that both sites can be cleaved by selective hydrolysis with carboxypeptidase Y. This selective modification of PF-4 by protesses could have great significance as it could affect the clearance of secreted PF-4 and its binding to endothelial cells, as well as its reactivity in radioimmunoassays. Studies are also planned to identify any endogenous proteases which might cleave and modify circulating PF-4 and also to identify altered forms of PF-4 in the circulation. The long-range goal of these studies is to determine whether PF-4 has an important physiologic role in hemostasis and thrombosis and to test whether it is a useful clinical marker for platelet activation in patients with thromboembolic disorders.